Hey, I just got the game and started a campaign with 3 exos on Easy. The difficulty description says you get 4 parts per mission, guaranteed. When I got 3 on my first mission, I thought it must mean you get 1 per exo, guaranteed. But then I did the second one, a Shieldworks, with 1 surviving exo, and got 2 parts. So what gives?

Usually it really is one piece per surviving exo. If you don't see this using a reduced exo count something is bugged. Did you by chance use Rey as your Pilot? His ability might be what throws the counts off...

EDIT: This is only the vanilla rule, though. There are other mission types (like Hostage) where you are receiving a piece of loot per achieved goal (rescued Cryogenic Pods in this case). AFAIK this is not the case for Shieldworks missions, though.